The Carbon Dioxide Enhanced Oil Recovery (CO2 EOR) application to the offshore oil field in Vietnam has been investigated through an international joint study scheme between Japan and Vietnam since 2007 and a preliminary study for potential CO2 EOR application indicated feasibility to some extent. To reduce and mitigate technical uncertainties/risks in the field scale EOR implementation, in May 2011, a CO2 EOR pilot test was executed in a sandstone reservoir of Rang Dong oil field, offshore Vietnam. The test was designed to be conducted as a single-well test, and generally called CO2 Huff-n-Puff. Major field operations included test string installation, pre-CO2 injection flow, CO2 injection (ca. 111 metric tonnes) and post-CO2 injection flow. Whole operation was performed satisfactorily with all objectives achieved. During the test, clear increment of oil rate and reduction of water cut were observed after CO2 injection as well as oil composition change. The contrast of CO2 injectivity and oil saturation change in the reservoir was confirmed by saturation logs, etc. Being the first CO2 EOR application in Southeast Asia and also offshore application worldwide is rarely reported, the operational gained is especially valuable. This paper is to describe the process of engineering design, planning, onshore preparation and field execution leading to the success of the project 1 . IntroductionRang Dong oil field is located approximately 135km offshore from Vung Tau, Vietnam. The field has been producing oil since 1998 from two major reservoirs, fractured granite Basement (BM) reservoir and Lower Miocene (LM) sandstone reservoir. From 2007 to 2010, a study of CO2 EOR application for sandstone reservoir in Rang Dong field had been implemented. The study result indicated that more than 32 million barrels of additional oil would be recovered; however it will require huge investment for construction of CO2 supply infrastructure, CO2 separation/ recovery, and the modification of existing production facilities, etc. Therefore, CO2 Huff-n-Puff was selected as the first step preparing for field scale CO2 EOR application in the future. CO2 Huff-n-Puff is basically a well stimulation technique in one well, which composes mainly three stages i.e., 1) Inject CO2 into a single producing well [Huff], 2) Shut in the well to allow CO2 to dissipate and dissolve, and 3) Produce the well back [Puff]. Selection of test well was one of critical issues for a successful pilot test in terms of reservoir quality to secure good CO2 injectivity and high reservoir pressure to achieve minimum miscible pressure (MMP) as defined from the study 2 during CO2 injection. The selected test well was an oil producer of BM reservoir, which had well path penetrating LM reservoir at shallower section. The BM reservoir of the well was no longer producing oil and under schedule for sidetracking. Therefore, it was proposed to perforate 9-5/8" casing of the well to access LM reservoir for CO2 Huff-n-Puff test. Fig.1 is the schematic of test well. The entire project...